Certain normal physiologic changes occur in every person who goes to altitude:

Hyperventilation (breathing faster, deeper, or both)

Shortness of breath during exertion

Changed breathing pattern at night

Awakening frequently at night

Increased urination

As one ascends through the atmosphere, every breath contains fewer and fewer molecules of oxygen. One must work harder to obtain oxygen, by breathing faster and deeper. This is particularly noticeable with exertion, such as walking uphill. Being out of
breath with exertion is normal, as long as the sensation of shortness of breath resolves rapidly with rest. The increase in breathing is critical. It is therefore important to avoid anything that will decrease breathing, e.g. alcohol and
certain drugs. Despite the increased breathing, attaining normal blood levels of oxygen is not possible at high altitude.

Credit to Dr. Tom Dietz of the ISMM for the original document edited to fit this webpage.